Secondary ion and electron yield measurements for surfaces bombarded with large molecular ions

Secondary-ion and secondary-electron emission yields from surfaces bombarded with large molecular ions have been measured in a tandem time-of-flight mass spectrometer. The primary ions were produced by matrix-assisted laser desorption/ionization and ranged in mass from about 6000 to 110000 u, and in energy from 5 to 25 keV. The yields were measured for surfaces of stainless steel and CsI in modest vacuum conditions. Electron and ion emission were observed for incident velocities as low as 3.5 km/s (0.05 eV/u). For a given energy, the yields decrease rapidly with increasing mass in the low mass range, but for large incident projectiles at 25 keV (or higher), the efficiency of secondary ion production is more or less constant near unity. The efficiency of secondary electron production continues to decrease slowly with increasing mass, but remains > similar to 30% throughout the mass range typically encountered in mass spectrometry of proteins (i.e. molecular weight < similar to 300 000 u corresponding to > 0.1 eV/u). For high velocities, the yield of electrons and ions is significantly higher for a CsI surface compared to stainless steel, but for velocities corresponding to < similar to 0.4 eV/u, the emission is rather insensitive to the surface.